1. Field of Invention
The invention relates generally to a subsea wellhead assembly. More specifically, the invention relates to an external tieback connector that may be disconnected from a subsea wellhead housing either with a remote operated vehicle, electrically via an umbilical, or acoustically with am acoustic transducer.
2. Description of Prior Art
One technique for drilling and producing offshore wells involves what is referred to as tieback connections. A floating platform drills and produces the wells. During drilling, the operator will install a subsea wellhead housing at the sea floor at the upper end of the well. An external tieback connector on a lower end of a string of drilling riser locks to an external profile on the wellhead housing. The drilling riser extends from the wellhead housing to a blowout preventer at the upper end of the drilling riser.
After the well has been drilled and the drilling riser removed, the operator installs an internal tieback connector inside the wellhead housing. The internal tieback connecter connects a production riser to the wellhead housing. The production riser extends up to the floating platform. A production tree will be installed on the upper end of the production riser for controlling well field produced from the well.
The conventional method for releasing a drilling riser and external tieback connector from the subsea wellhead housing uses a remote operated vehicle (ROV). The operator deploys the ROV from the floating platform on an ROV umbilical. The ROV engages an ROV interface on the tieback connector, then injects hydraulic fluid under pressure from the ROV into the tieback collector to release the tieback connector locking element from the subsea wellhead housing.
During the drilling process, an emergency may occur in which the operator needs to quickly release the tieback connector and the drilling riser from the subsea wellhead housing. While releasing can be performed with an ROV, it might take two or more hours to deploy an ROV from the floating platform and perform the releasing procedure.
Emergency systems exist for subsea well drilling techniques that do not use an external tieback connector. Rather than connecting a tieback connector to a subsea wellhead housing, a large, complex blowout preventer (BOP) connects to the subsea wellhead assembly. The BOP has rams that may be closed in an emergency. A release mechanism disconnects the drilling riser and upper part of the BOP from the lower part containing the rams. An umbilical extends from the BOP to the drilling platform for performing these emergency steps. The BOP has accumulators with valves that when open deliver hydraulic fluid snider pressure to perform these and other functions. Some subsea BOPs have alternate ways to close rams and release the riser in the event of problems with the umbilical, such as techniques using ROV's and/or acoustic transducers.